Resolving Low Cloud Feedbacks Globally with HR-MMF: Agreement with LES but Stronger Shortwave Effects

This study uses the Multiscale Modeling Framework (MMF) of the Energy Exascale Earth System Model (E3SM-MMF)  to explore low cloud feedbacks. E3SM-MMF embeds a two-dimensional Cloud-Resolving Model (CRM) based on the System for Atmospheric Modeling (SAM) within each column of a host Global Climate Model (GCM). The CRM employs a one-moment microphysics scheme, Smagorinsky turbulence closure, and the Rapid Radiative Transfer Model for GCM Applications-Parallel (RRTMGP) for radiative transfer, optimized for GPU performance. CRM columns are grouped for radiative calculations to reduce computational costs without affecting solution quality.

The simulations use an optimized E3SM-MMF variant with enhancements such as horizontal hyperviscosity, cloud droplet sedimentation, refined autoconversion thresholds for liquid (qcw0qcw0 = 5 × 10−4) and ice (qci0qci0 = 5 × 10−5), and non-local CRM variance transport. The CRM grid has 200 m horizontal resolution (Δx) with 256 columns (51.2 km domain), while the GCM uses a ``ne30pg2'' horizontal grid (~1.5∘ resolution) and a 125-level vertical grid (L125), with Δz = 20 m resolution between 500–1,800 m to resolve marine stratocumulus inversions. A 2 s CRM time step ensures numerical stability.

Two 5-year simulations were conducted: a control (0 K) experiment with default NOAA SST and sea ice data, and a warmed (4 K) experiment with SST uniformly increased by 4 K. Both simulations used identical configurations, ensuring reasonable low cloud amounts in the control climate. The latest E3SM version is available at \url{https://github.com/E3SM-Project/E3SM.git}.